If the -ing form is used as an adjective (an interesting book) or as a main verb (I was watching TV all night), it's a present participle.
1 Subject: Smoking kills. (kills = main verb and smoking is the subject- you could change it to a noun and it would have the same function: tobacco kills.
2 Position- if it comes after a preposition
Are you clear on what these terms mean? If you have I like reading, what would you call reading?
Student or Learner